In general, a conventional drive control system for a hydraulic construction machine comprises a prime mover, a hydraulic pump driven by the prime over, a hydraulic actuator driven by discharge hydraulic fluid from the hydraulic pump, rotational speed setting means including a fuel lever for setting rotational speed of the prime mover, and an operating lever for controlling operation of the hydraulic actuator. Connected between the hydraulic pump and the hydraulic actuator is a control valve for controlling flow rate and direction of the discharge hydraulic fluid from the hydraulic pump. Operation of the operating lever controls the position of the control valve to control operation of the hydraulic actuator.
In the above conventional system, rotational speed of the prime mover or an engine is set by displacement of the fuel lever, to vary a horsepower characteristic of the engine in accordance with the set rotational speed. The maximum horsepower of the engine is determined on the basis of the horsepower characteristic. Specific fuel consumption (g/PS.h) of the engine is determined depending upon the set rotational speed and the magnitude of an operational load at that time. If, for example, the rotational speed is set to the maximum value, the specific fuel consumption is brought to the best value, at the heavy load operation in the vicinity of the maximum horsepower of the horsepower characteristic obtained by the set rotational speed. On the other hand, at the light load operation which requires only horsepower lower than the maximum horsepower, the engine rotational speed increases to a value higher than the rotational speed at the maximum horsepower of the horsepower characteristic, so that the specific fuel consumption is deteriorated. In general, at the actual operation of the hydraulic excavator, for example, the proportion of the operation which is carried out under the load excellent in the specific fuel consumption is extremely low. For example, in one operational cycle including excavating, boom raising-swing, dumping and boom lowering swing which are repeated in the mentioned order, the operations necessitating the above-mentioned maximum horsepower are only the relief excavating during the operation and the operation at acceleration at the initial stage of the swing during the operation Thus, it is not preferable from the energy-saving point of view to set the rotational speed to the maximum value.
In the drive control system of the kind referred to above, Japanese Patent Application Laid-Open No. 52-53189 has proposed an arrangement in which not only is the rotational speed of the engine set by the fuel lever, but also the engine rotational speed is interlocked with the operating lever for controlling operation of the hydraulic actuator in such a manner that when the operating lever is operated, the engine rotational speed is set also by displacement of the operating lever, to vary the horsepower characteristic, thereby controlling the maximum horsepower. With this arrangement, when displacement of the operating lever is small, the engine rotational speed is set to a low value to give the maximum horsepower required for the light load operation, while as the displacement increases, the engine rotational speed is set to a high value to raise also the maximum horsepower of the engine so as to give the maximum horsepower required for the heavy load operation. Thus, the operation is carried out always in a region excellent in the specific fuel consumption, thereby preventing deterioration of the specific fuel consumption. Further, in a similar drive control system, Japanese Patent Application Laid-Open No. 58-204940 has proposed an arrangement in which only a specific operating lever is interlocked with the engine rotational speed, and only when the operating lever is operated, the engine rotational speed is set by displacement of the operating lever to vary the horsepower characteristic, thereby controlling the maximum horsepower. In this system, low rotational speed providing the maximum horsepower required for the light load operation is set by the fuel lever and, usually, the operation is carried out with the horsepower characteristic obtained at the low set rotational speed. When the specific operating lever is operated, the rotational speed is set, in interlocked relation to the operation of the operating lever, to a value higher than that set by the fuel lever, so as to give the maximum horsepower required for the heavy load operation with the horsepower characteristic obtained at the set rotational speed, like the above-described conventional system. Thus, the operation is carried out always in a region excellent in the specific fuel consumption, thereby preventing deterioration of the specific fuel consumption.
Furthermore, in the drive control system described above, Japanese Patent Application No. 59-129957 has proposed an arrangement comprising, in place of the control valve, a hydraulic pump of variable capacity type and means for varying an angular position of a swash plate of the hydraulic pump, that is, a displacement volume of the hydraulic pump by the operating lever, wherein the engine rotational speed is controlled only by the operating lever, the engine rotational speed is set to a low value when displacement of the operating lever is equal to or lower than a predetermined value, and as the displacement of the operating lever exceeds the predetermined value, the rotational speed is set to a high value in dependence upon the displacement of the operating lever, also in this system, like the above-mentioned conventional system, an attempt can be made to improve the specific fuel consumption, because, in the displacement of the operating lever equal to or larger than the predetermined value, the engine rotational speed is set on the basis of the displacement of the operating lever.
In addition to the above-mentioned patent applications, Japanese Patent Application Laid-Open Nos. 48-53162 and 50-15980, and Japanese Patent Publication No. 60-38561 are listed as being relevant to the arrangement in which the engine rotational speed is interlocked with operation of the operating lever. Moreover, U.S. Pat. Ser. No. 947,524 (corresponding to EPC Application No. 86118113.9) discloses an arrangement in which the engine rotational speed is controlled in response to operation modes or the actuator load.
In the system disclosed in Japanese Patent Application Laid-Open Nos. 52-53189 and 58-204940, however, operation of the operating lever setting the engine rotational speed by the operating lever is effected substantially over the entire range of the operating lever. Accordingly, each time displacement of the operating lever varies by operation thereof, the set rotational speed varies, so that the engine rotational speed frequently fluctuates during almost all of a period of time within which the operating lever is operated. In, for example, the above-mentioned operational cycle, when the fuel lever is operated to set the rotational speed to a low value suitable for the operation which is minimum in the requisite horsepower, operation of the operating lever causes the engine rotational speed to frequently fluctuate at the operations other than the operation. This requires power for accelerating a flywheel of the engine, resulting in consumption of the fuel. Thus, there has been a problem that the specific fuel consumption is not necessarily improved. Further, there have also been problems relating to smoke emission and noises which occur due to fluctuation of the engine rotational speed.
Moreover, the system disclosed in Japanese Patent Application Laid-Open No. 58-204940 has the following problem. That is, when an operating lever other than the specific operating lever is operated, the rotational speed set by the fuel lever is low and, therefore, it is impossible to carry out the operation necessitating the output power equal to or higher than the maximum horsepower obtained with the horsepower characteristic at the set rotational speed. Thus, a bad influence is exerted upon the operability. Specifically, for example, in the above-mentioned operational cycle, when an operating lever carrying out the boom raising-swing operation is selected as the specific operating lever, the requisite maximum horsepower cannot be obtained at the relief excavating of the operation In other words, it is impossible for the operating lever other than the specific operating lever to effectively utilize the maximum horsepower of the engine.
Furthermore, in the system disclosed in Japanese Patent Application No. 59-129957, the arrangement is such that the engine rotational speed is set to a constant low value in a region of operation of the operating lever equal to or less than the predetermined displacement. Since, however, the constant value is fixedly determined, the operating lever must be operated with displacement equal to or larger than the predetermined value to set the engine rotational speed to a higher value, at the operation necessitating the maximum horsepower higher than that obtained with the horsepower characteristic of the set low rotational speed. Also in this case, the engine rotational speed frequently fluctuates, giving rise to problems such as deterioration of the specific fuel consumption, smoke emission and generation of noises. For example, in the above-mentioned operational cycle, when the constant rotational speed is set to a low value suitable for the operation lowest in the requisite horsepower, operation of the operating lever causes the engine rotational speed to frequently fluctuate at the operations other than the operation This raises problems such as deterioration of the specific fuel consumption due to acceleration of the flywheel, smoke emission and noises. In addition, when the constant rotational speed is set to a high value, the engine rotational speed is brought to a high value inferior in the specific fuel consumption from the horsepower characteristic point of view, at the operation which necessitates only the horsepower lower than the maximum horsepower obtained with the horsepower characteristic of the constant rotational speed, thereby making it impossible to achieve the original object. That is, in the above-mentioned operational cycle, when the constant rotational speed is set to an intermediate value suitable for the usual excavating operation of and the swing operation subsequent to the initial acceleration of , the specific fuel consumption is deteriorated at the operations and which are low in requisite horsepower.
Furthermore, since the constant rotational speed is determined in a fixed fashion, even if an operator desires operation in which noises and smoke emission due to fluctuation of the rotational speed are not caused, it is impossible to carry out such desired operation. Thus, there has been a problem regarding the operability.